1. Technical Field
The subject matter described here generally relates to fluid reaction surfaces with specific blade structures that are formed with a main spar, and, more particularly, to wind turbine blade spars having jointed shear webs.
2. Related Art
A wind turbine is a machine for converting the kinetic energy in wind into mechanical energy. If that mechanical energy is used directly by machinery, such as to pump water or to grind wheat, then the wind turbine may be referred to as a windmill. Similarly, if the mechanical energy is further transformed into electrical energy, then the turbine may be referred to as a wind generator or wind power plant.
Wind turbines use one or more airfoils in the form of a “blade” to generate lift and capture momentum from moving air that is them imparted to a rotor. Each blade is typically secured at its “root” end, and then “spans” radially “outboard” to a free, “tip” end. The front, or “leading edge,” of the blade connects the forward-most points of the blade that first contact the air. The rear, or “trailing edge,” of the blade is where airflow that has been separated by the leading edge rejoins after passing over the suction and pressure surfaces of the blade. A “chord line” connects the leading and trailing edges of the blade in the direction of the typical airflow across the blade.
Wind turbines are typically categorized according to the vertical or horizontal axis about which the blades rotate. One so-called horizontal-axis wind generator is schematically illustrated in FIG. 1. This particular configuration for a wind turbine 2 includes a tower 4 supporting a drive train 6 with a rotor 8 that is covered by a protective enclosure referred to as a “nacelle.” The blades 10 are arranged at one end of the rotor 8 outside the nacelle for driving a gearbox 12 connected to an electrical generator 14 at the other end of the drive train 6 inside the nacelle.
The blades 10 for such wind turbines 2 are typically fabricated by securing various “shell” and/or “rib” portions to one or more “spar” members extending spanwise along the inside of the blade for carrying most of the weight and aerodynamic forces on the blade. Spars are typically configured as I-shaped beams having a web, referred to as a “shear web,” extending between two flanges, referred to as “caps” or “spar caps,” that are secured to the inside of the suction and pressure surfaces of the blade. However, other spar configurations may also be used including, but not limited to “C-,” “L-,” “T-,” “X-,” “K-,” and/or box-shaped beams. The shear web may also be utilized without caps.
For example, one so-called “box-spar” blade configuration with forward and aft shear webs extending between the ends of two spar caps is illustrated in the U.S. Department of Energy, National Renewable Energy Laboratory, Publication No. NREL/SR-500-29492 (April 2001). Commonly-assigned and co-pending U.S. patent application Ser. No. 11/684,230 filed on Mar. 9, 2007 by Alhoff et al. discloses various other configurations, including web portions that are adhesively bonded to and/or integrated with shell portions of the blade. In one embodiment, an adhesive joint is disposed between web portions extending from two integrated shells. Other embodiments include spar caps which are adhesively joined to shell portions and/or web portions. The adhesive joints may also include incorporation into the shell portions by matrix infusion.
However, such conventional approaches have been found to suffer from a variety of drawbacks. For example, turbine blade shells and spars must often be manufactured with large dimensional tolerances. Those tolerances can then accumulate to create wide gaps where the parts are joined together and/or joined with other parts of the blade 10. As illustrated in the schematic cross section of the wind turbine blade 10 that is shown in FIG. 2, any dimensional discrepancies between the shell 20 and the spar 22 will increase the amount of bonding material 24 that is required in order to fill the resulting gaps between the shell and the spar and/or the gaps between the shear web 26 and/or the spar caps 28. Such thick bond lines can add substantial weight and stress to the blade. Moreover, these low-strength, high-weight bond lines are located just where stresses on the blade are likely to be highest.